Radiocesium dynamics in the aquatic ecosystem of Lake Onuma on Mt. Akagi following the Fukushima Dai-ichi Nuclear Power Plant accident.

Abstract Understanding ecosystem dynamics of radionuclides is necessary to ensure effective management for food safety. The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident on March 11, 2011 released large amounts of radiocesium ( 134 Cs and 137 Cs) and contaminated the environment across eastern Japan. In this study, we aimed to elucidate the temporal dynamics of 137 Cs in the aquatic ecosystem of Lake Onuma on Mt. Akagi. The effective ecological half-life ( T eff ) of 137 Cs in fishes, western waterweed ( Elodea nuttallii ), seston (phytoplankton and zooplankton), and lake water was estimated using survey data of 137 Cs concentration collected from 2011 to 2016, and single- and two-component decay function models (SDM and TDM, respectively). The decay processes of 137 Cs concentrations in wakasagi ( Hypomesus nipponensis ), pale chub ( Zacco platypus ), phytoplankton, and total 137 Cs concentrations of the water column (WC) in the lake were well suited by the TDMs. The T eff in the fast component of the TDMs in these samples ranged from 0.49 to 0.74 years. The T eff in the slow component of the TDMs could converge towards the physical half-life of 137 Cs. Nearly five and a half years after the FDNPP accident, we concluded that 137 Cs concentrations approached a state of dynamic equilibrium between some aquatic organisms (wakasagi, pale chub, and phytoplankton) and the environment (lake water). However, the decay processes of 137 Cs concentrations in Japanese dace ( Tribolodon hakonensis ), western waterweed, zooplankton, and particulate- and dissolved-forms in the WC were better predicted for the SDM. The total 137 Cs concentrations in inflowing river and spring waters were one to two orders of magnitude lower than lake water under normal flow conditions. However, particulate 137 Cs contamination level in the river water was high after heavy rains. Overall, 137 Cs contamination levels have significantly decreased in Lake Onuma, but monitoring surveys should be continued for further understanding of the reduction processes.